The Loves of Cynthia (1972)

Cynthia has to leave her modeling career in Rome (no longer wanted) and comes to stay with her sister, Tess who married a millionaire, Edward who is her ex-lover. While there she devises a plan for Edward's brother Philip who hates him because he runs the family business and stableman to have sex with her sister after being drugged with an aphrodisiac in the hopes that her Edward will divorce her sister and marry her.